ESSnuSB is a project introduced by the IPHC (M. Dracos) and the University of Uppsala (T. Ekelöf). It follows the EUROnu Design Study FP7 (2008-2012). The aim is to produce very intense neutrino beams using the protons from the ESS linac (5 MW, 2 GeV) and a 4-targets horn system. In the ESSnuSB proposed facility a copious number of muons will also be produced. These muons could be used by a future Neutrino Factory to study CP violation in the leptonic sector but also to study neutrino cross-sections. They could also be used to feed a future muon collider.

ESSnuSB is supported by the COST Action CA15139 "Combining forces for a novel European facility for neutrino-antineutrino symmetry-violation discovery" (EuroNuNet). 13 countries already joined the project. It has been recently granted from the H2020 INFRADEV program for a 4-years Design Study. A kick-off meeting is foreseen in January 2018.

The ESSνSB (standing for European Spallation Source Neutrino Super Beam) project proposes to study a Super Beam which uses the high power linac of the ESS facility [1] based at Lund in Sweden as a proton driver and a MEMPHYS type detector [2, 3] located in a deep mine at a distance of about 500 km, near the second neutrino oscillation maximum (Fig. 1).

Figure 1: ESSνSB layout on top of the ESS neutron facility.

The ESS Linac

ESS will deliver a first proton beam for neutron production at reduced energy and power by 2019. A proton beam of the full design power 5 MW and energy 2.0 GeV will be delivered by 2023 (Table 1). There will be 14 pulses of 62.5 mA current and 2.86 ms length per second. In order for the ESS to be used to generate a neutrino beam in parallel with the spallation neutrons, some modifications of the proton linac are necessary. A preliminary study of these modifications that are required to allow simultaneous acceleration of H+ (for neutron production) and H− (for neutrinos) ions at an average power of 5 + 5 MW has been made [4].

Table 1: Main ESS facility parameters [1]

The accumulator ring and beam switchyard

An accumulator ring to compress the pulses to 1.5 µs is mandatory to avoid overheating issues of the neutrino targets. A first estimation gives a ring having a circumference of 376 m [5] (Table 2). Each pulse from the ESS linac will contain 1.1x1015 protons, which for a normalized beam emittance of 200 π mm mrad in the ring by multi-turn injection (the emittance from the linac should be in the order of a few mm-mrad) will lead to the space-charge tune shift of about 0.75.

Table 2: Accumulator parameters [5]

The H- ions will be fully stripped during the injection into the accumulator using either stripping foils or a laser-stripping device [5, 6]. The extraction of the beam from the ring needs a group of kickers that should have a rise time of not more than 100 ns.
Four separate targets are needed in order to mitigate the high power dissipation in the target material. A beam switchyard system downstream the accumulator ring will distribute the protons onto the targets [7].

The horn/target station

The target station includes the target itself that is hit by the protons leading to the production of short-lived mesons, mainly pions, which decay producing muons and muon neutrinos. A packed bed of titanium spheres cooled with pressurized helium gas has become the baseline target design for a Super Beam based on a 2-5 GeV proton beam with a power of up to 1.2 MW per target. The packed bed concept has been studied using Computation Fluid Dynamics (CFD) software tools.

Figure 2: Horn layout (the targets are inside the horns).

Other main components of the target station are the hadron collector called magnetic horn (Fig. 2), which focuses the hadrons towards the far neutrino detector, and the decay tunnel, long enough to allow the mesons to decay, but not as long as to allow for a significant amount of the muons to decay. In order to mitigate the detrimental effects of the very high power of the proton beam hitting the target, EUROν [8] has proposed a system with four targets and horns, sharing the full beam power between the four. This system will be adopted here.

Underground detector site

The Northern Garpenberg mine, located at 540 km NW of the ESS site in Lund in Sweden, is one of the candidate mines that could host the large underground Water Cherenkov detector. This mine is being studied in detail collecting geological and rock mechanics information at potential detector locations, situated at 1000 m depth (3000 m water equivalent) and at least 500 m from locations with active mining operations, by making core drillings, core logging, rock strength testing and rock stress measurements of the surrounding rock.
Once a suitable location for the neutrino detector underground halls has been determined (total volume of 6x105 m3), a design of the geometry and construction methods for the underground halls will be made based on the measured strength and stress parameters of the rock.

Physics Potential

According to first evaluations [9], for which 5% systematic error on the signal and 10% systematic error on the background were assumed, leptonic CP violation could be discovered at 5 σ confidence level within at least 50% of the CP phase range for baselines in the range 300-550 km with an optimum of about 58% of the phase range at a baseline of about 420 km, already a very competitive physics performance [10]. According to the same first evaluations, the neutrino mass hierarchy can be determined at more than 3 σ confidence level for baselines in the range 300–500 km depending on the proton beam energy.

In addition, inclusion of data from atmospheric neutrino oscillations in the mass hierarchy determination will certainly improve the physics reach of this project.

Muon Production

In addition to the neutrinos, the ESSnuSB proposed facility will produce a copious number of muons at the same time. 2.7x1023 protons are foreseen to hit the targets within one-year operation. Preliminary studies show that 3.5x1020 pions and 4.2x1022 muons per m2 and per year will be available at the level of the beam dump which is located 25 m after the horn-target system [11, 12]. Figure 3 presents the impacts of remaining pions and produced muons at the surface of the beam dump.

Figure 3: Pions (a) and muons (b) at the surface of the beam dump (normalized by proton).

The produced muons could be used by a low energy nuSTORM [13] facility to measure neutrino cross-sections at the energies where this neutrino facility will be operated. They could also be useful for 6D muon cooling experiments and in an ultimate stage they could be used to operate a Neutrino Facility or a muon collider.

Figure 4: Momentum distribution: pions (a); muons (b).

The mean value of the momentum of pions and muons is 0.7 GeV and 0.46 GeV, respectively (Fig. 4). For these energies, the mean free path of pions is of the order of 40 m after which they will decay to give some more muons. The mean free path for the muons is 2.9 km which is enough to send them in a ring, as the one foreseen for nuSTORM, where they can decay in straight sections to produce muon and electron neutrinos to be used to measure cross-sections.
While for nuSTORM muon beam an iron absorber is needed to lower the muon momentum to a mean value of 400 MeV in order to perform 6D muon cooling experiments (of which success could lead to the construction of a Neutrino Factory and of a muon collider), for ESSnuSB the muon momentum is directly around the required values. nuSTORM plans to collect in the region between 200 MeV/c and 500 MeV/c about 4.3x1017 muons per year while the ESSnuSB facility could provide more than 2.5x1020 muons per year for the same momentum range.

IPHC/Neutrino implications:

Strong from its expertise gained during the EUROnu project, the neutrino team is implicated in:
the estimation of the muons/pions/neturinos produced by the horn and the optimization of this latter,
the radioprotection studies along the tunnel,
the design of beam lines (muon extraction, beam switchyard, beam extraction from the accumulator),
the feasibility of the power supply of the target station…